A known sound insulation part of this type (DE-GM 82 01 511) has at least one heavy layer and a springy layer between the heavy layer and the corresponding part of the motor vehicle or the like, such as a body panel, having grooves and/or depressions and/or knobs formed therein for the purpose of forming discontinuities. The depressions are preferably cup-shaped.
The purpose of such sound insulation parts is to achieve high noise comfort with the lowest possible weight.
For physical reasons, however, the possibilities of saving in mass with simultaneous noise reduction are limited with the known mass-spring systems, i.e. parts that are too light can no longer fulfil the requirements for noise comfort. Attempts have already been made to construct sound insulation parts of large surface area in such a way that parts of the surface with greater sound yield are equipped more intensely acoustically, e.g. through mass coating or proofing against solid-borne sound (cf DE-AS 27 32 483). According to another approach, sound sinks are subjected to a more intense treatment, such that the intensity vector of the sound radiation (corresponding to a sink), which is directed outwards from the inner passenger space, is augmented (cf DE-GM 83 36 676) by applying an additional soft spring to the sound sinks. These measures do not allow any saving in weight, so that the requirements for series production are still not optimal.
More recent investigations on sound fields in passenger cabins, in particular of cars, have been made primarily by consistently using and further developing the methods for measuring sound intensity or for measuring sound energy flux. With the help of such methods the source-sink distribution in a vehicle can be determined (cf in particular Kutter-Schrader, H., Betzhold, Ch. and Gahlau, H. "Intensitatsmessung im Kraftfahrzeuginnenraum mit einem kleinen Analogmessgerat" [Intensity measurement in motor vehicle interiors with a small analogue measuring instrument], VDI-Report, 526, p 137-151). These methods have further indicated how the vectorially directed sound energy flux emerging from the roof of a vehicle can be treated, by solid-borne sound proofing measures, so that the intensity vectors are markedly reduced and the disturbing low-frequency resonance vibration no longer occurs.
Proceeding from these considerations, it appears that starting-points for providing effective sound insulation using as small a mass as possible are: if the parts as a whole are light in weight, to provide them at the sound radiators with additional means, such as a thicker heavy layer, only on the parts of the surfaces recognised as to be treated, In particular, effective sound insulation can be expected if an alteration in the source-sink distribution is brought about such that the distribution of the sources and sinks obtained after the sound insulation measure is as uniform as possible, with strong sinks directly adjacent to the strong sources. With motor vehicle bodies it has, however, been found that powerful sound radiators (sources) must be converted into sinks.
With mass-spring systems of the kind mentioned it is known to achieve weight savings if the spring has gas-filled hollow chambers (cells) which are totally or partially enclosed by foil, these cells being distributed symmetrically over the whole surface of the sound insulation part (cf DE-OS NO. 27 50 439 and DE-GM 79 29 637). Through the uniform and symmetrical arrangement of the gas-filled cells, the whole spring becomes stiffer since gas-filled closed chambers become incompressible.